Proxy caching in a photosharing peer-to-peer network to improve guest image viewing performance

ABSTRACT

The present invention provides a method and system for serving an image stored in the peer computer to a requesting computer in a network photosharing system in which the peer computer is coupled to a photosharing system server. Aspects of the invention include caching copy of the image in the photosharing server; and in response to the photosharing server receiving a request from the requesting computer to view the image stored in the peer computer, transmitting the cached image from the photosharing server to the requesting computer, thereby avoiding the need to transfer the image from the peer computer to the photosharing server for each request to view the image.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation-in-part of co-pending patent application Ser. No. 10/813,839, filed Mar. 31, 2004, entitled “Method and System for Providing Web Browsing Through a Firewall in a Peer to Peer Network” (P224/3029P), and herein incorporated by reference.

FIELD OF INVENTION

The present invention relates to sharing digital images over a network, and more particularly to a method and system for improving guest viewing performance of images stored on peer computers in a photosharing peer-to-peer network.

BACKGROUND OF THE INVENTION

Over the past several years, photosharing has become widely accepted by photo enthusiasts. Many websites currently exist that allow users to upload digital images to the site for storage on a server and for viewing by others over the Internet. Oftentimes, images are grouped to form an album web page so that the user can invite others to view the album web page, as opposed to each individual image.

This approach to photosharing, however, presents some disadvantages. In particular, users are required to upload digital images to the site, which can be time consuming, and the site requires extensive storage capacity to store the images of all the users, which can become expensive.

To address these concerns, the assignee of the present invention has developed a web-based peer-to-peer photosharing system in which all workstations and computers (peers) in the network store images locally and act as servers to other users on the network. A central site accessible by all the peers provides additional functionality, such as coordinating the peers, providing search capabilities for the peers, purchase order fulfillment, etc.

FIG. 1A is a block diagram illustrating the web-based peer-to-peer photosharing system. The peer-to-peer photosharing system 20 includes a photosharing P2P network 22, which comprises multiple peer servers 24 running peer node software 26 and web server software 28. The peer node and server software 24 and 26 enable the users of the computers to share images with others in the network 22 through a web browser 30 without having to upload their pictures to a web site. A novel feature of the photosharing P2P network 22 is that it provides a hybrid peer-to-peer architecture for general HTTP/web browser configuration that incorporates a central proxy server 36 to coordinate networking traffic for peers behind firewalls, thus allowing access to peers behind firewalls by other peers and by visiting computers 32 not in the network 22. The proxy server 36 provides supporting services to the peers 24 as well as providing a path through which the visiting computer 32 accesses images from the peer servers 24 via a standard web browser 30. Allowing generic HTTP access to the images hosted on peer servers located behind firewalls becomes increasingly important as virtually all corporations use firewalls, and the use of software firewalls installed on user's home systems is becoming ubiquitous.

FIG. 1B is a diagram illustrating the data paths used when an image is served from one of the peers 24 to a visiting computer 32 through the proxy server 36. The process begins when a guest initiates a request from the web browser 30 of the visiting computer 32 to view an image, which is routed to the proxy server 36 via path (A). The proxy server 36 then routes the request to the peer server 24 via path (B). The peer server 24 services the request and returns the image to the proxy server via path (C). The proxy server 36 then sends the image to the visiting computer 32 for display via path (D).

One problem with routing images through the proxy server 36 is that it requires extra bandwidth. That is, the image must be moved twice; once from the peer server 24 to the proxy server 36, and then a second time from the proxy server 36 to the visiting computer 32. In addition, path (C) generally has the longest latency because the peer server 24 is typically hosted in a user's home equipped with a cable modem or DSL that has an uplink speed (path C) much slower that the downlink speed (path B). Thus, users may experience a significant delay when viewing images that are routed through the proxy server 36 due to the bandwidth limitations between the peer server 24 and the proxy server 36 path (C).

Accordingly, there is need for a method and system for reducing the amount of network traffic between the peer server and the proxy server (path (C)) in order to improve guest image viewing performance. The present invention addresses such a need.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and system for serving an image stored in a peer computer to a requesting computer in a network photosharing system in which the peer computer is coupled to a photosharing server, such the proxy server described above. Aspects of the invention include caching copy of the image in the photosharing server; and in response to the photosharing server receiving a request from the requesting computer to view the image stored in the peer computer, transmitting the cached image from the photosharing server to the requesting computer, thereby avoiding the need to transfer the image from the peer computer to the photosharing server for each request to view the image. In a second aspect of the present invention, prior to serving the cached image to the requesting computer, the photosharing server determines whether the cached image has been modified since being cached by sending an HTTP request with a timestamp of the cached image to the peer computer. The peer computer then compares the timestamp of the cached image with the timestamp of the image stored on the peer, and sends a response to the photosharing server indicating whether the image has been modified based on whether the timestamps match or not. If the image has been modified, then the modified image is cached on the photosharing server and served to the requesting computer. In another aspect of the present invention, the images stored on the peer computer are automatically synchronized with the images on the photosharing server so that the photosharing server always serves the most up-to-date version of the image to requesters.

According to the method and system disclosed herein, caching the image in the photosharing server avoids the need to transfer the image from the peer computer to the photosharing server (path (C)) for each request to view the image. Thus, the present invention greatly reduces network traffic between the peer computer and the photosharing server and increases image viewing performance of the photosharing network.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram illustrating an exemplary web-based peer-to-peer environment.

FIG. 2 is a detailed block diagram of an improved photosharing network according to a preferred embodiment of the present invention.

FIG. 3 is a flow diagram illustrating the process for improving guest viewing performance of images stored on peer servers in a photosharing peer-to-peer network.

FIG. 4 is a flow diagram illustrating the process of a peer server registering with the photosharing peer-to-peer network to make its serving capabilities assessable through a firewall.

FIG. 5 is a diagram illustrating components of the proxy server and the flow between the requesting web browser, the proxy server, and the peer server to enable the web browser to have HTTP access to the peer server through the proxy server.

FIG. 6A is a diagram illustrating the contents of a peer request packet.

FIG. 6B is a diagram illustrating the contents of a peer response packet.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to sharing digital images over a network, and more particularly to a method and system for improving guest image viewing performance. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. For example, although the preferred embodiment is in the context of a peer-to-peer network, the same principles can be applied to a client-server environment where the guest browser communicates directly with the computer system storing the album and images. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein.

The present invention provides a method and system for improving the viewing performance of a peer-to-peer photosharing network in which peer servers store their user's images and are coupled to at least one photosharing system server, such as a proxy server.

FIG. 2 is a detailed block diagram of an improved photosharing network according to a preferred embodiment of the present invention. Besides the components the shown in FIG. 1A, each peer server 42 includes a peer node application 44 that functions according to the present invention, and a database 46 of stored images 48. The images 48 are typically stored on a hard drive of the hosting computer system, and are given a timestamp 50 by the host operating system that indicates the date of creation or modification date, as is well-known in the art.

The proxy server 40 acts as a proxy for the distributed peer servers 42, which have a pre-established connection to the proxy server 40. The proxy server 40 enables a firewall-protected peer server 42 to enable incoming generic HTTP access by establishing an outbound connection from the firewall-protected peer server 42 with the proxy server 36. Incoming Web traffic for the firewall-protected peer server 42 is then directed to the proxy server 40. The proxy server 40 multiplexes the Web traffic using a proprietary protocol to the peer server 42, thus enabling generic web traffic to flow to the peer server 42 despite the presence of a firewall (not shown). In the case where there are multiple firewall-protected peer servers 42, the proxy server 40 acts as a switchboard to receive and dispatch the incoming HTTP requests to the appropriate peer servers 42. The process for providing Web browsing through a firewall in a peer-to-peer network is described further detail with respect to FIGS. 4 through 6B. As used herein, the peer servers 24, proxy server 36 and the visiting computer 32 may comprise any computing device, such as PCs, workstations, mobile phones, and PDAs, with components necessary for executing the appropriate software. Also, in a preferred embodiment, the physical communications network is the Internet, although any type of network could be used.

According to the present invention, images 40 stored on the peer server 42 that are requested for viewing by a visiting computer 32, are stored in a cache 52 on the proxy server 40 either before or after the first time the image is requested. Thereafter, subsequent requests for the images are served from the proxy server's cache 52, rather than being fetched from the hosting peer server 42, thus increasing viewing performance. In addition, the proxy server 42 ensures that the cached images 48′ are fresh by sending a request to the peer server 42 to check whether a requested image 48′ has been modified since being cached. This is accomplished by comparing the timestamp 50′ of the cached image 48′ with the timestamp 50 of the image 40 stored on the peer server 42. If the comparison of the timestamps indicates that the image 48 on the peer server 42 has been modified, then the peer server 42 transfers the modified image to the proxy server 42 prior to the proxy server 42 serving the image to the requester.

By caching the images 48′ on the proxy server 40, the present invention significantly reduces the need to transfer the image 40 from the peer server 42 to the proxy server 40 along path (C) to satisfy each image request, thereby improving the viewing performance of the network. Although a small amount of data is transferred between the proxy server 40 and the peer server 42 in order to determine if the proxy's cache 52 is still fresh, this traffic is generally only a few bytes, as opposed to the kilobytes or megabytes required to transfer images 40 from the peer server 42 to the proxy server 40 for every image request.

FIG. 3 is a flow diagram illustrating the process for improving guest viewing performance of images stored on peer servers in a photosharing peer-to-peer network. Referring to both FIGS. 2 and 3, the process begins in step 60 when the visiting computer 32 issues a request to view the image 48 stored in the peer server 42. The request is routed to the proxy server 40 via path (A). In step 62 the proxy server 40 checks if a copy of the image 48′ is stored in the cache 52 via path (E). If the copy of the image 48′ is not stored in the cache (a cache miss), which may occur the first time an image is requested, then in step 64 the proxy server 40 sends a request for the image to the peer server 42 hosting the requested image 48 via path (B).

In step 68, the peer server 42 retrieves the image 48 and transmits a copy of the image 48 to the proxy server 40 via path (C). In step 70, the copy of the image 48′ is stored in the cache 52. In step 72, the proxy server 40 retrieves the cached image 48′ via path (F), and serves it to the requesting visiting computer 32 via path (E). In a preferred embodiment, the proxy server 40 streams the image 48′ to the visiting computer 32 while the image 48′ is being downloaded from the peer server 42 to further reduce the latency between the time that the request is made and the image is returned.

Referring again to step 62, if the image 48′ is present in the cache 52 (a cache hit), the proxy server 40 determines if the cached image 48′ is still fresh in step 74 by sending a request to the peer server 42 in the form of a standard HTTP “If-Modified-Since” header with the timestamp 50′ of the cached image 48′ via path (B). In step 76, the peer node application 44 on the peer server 42 compares the timestamp 50′ of the cached image 48′ with the timestamp 50 of the image 40 stored on the peer server 42. If the timestamp 50 of the image 48 stored on the peer server 42 is different (i.e., newer) than the timestamp 50′ of the cached image 48′, then the peer server 42 determines that the image 48 on the peer server 42 has been modified since the image was cached (stale cache). In step 68, the peer server 42 returns a copy of the image 48 as a response via path (C).

If the peer server 42 determines that the image 48 it has stored locally does not having newer timestamp 50 than the timestamp 50′ sent by the proxy server 40 in step 76, then in step 78, the peer server 42 sends a 304 HTTP return code to the proxy server 40 as a response indicating that the image has not been modified via path (C). The proxy server 40 then retrieves the image 48′ from the cache 52 via path (F) and serves it to the visiting computer 32 via step 72 via path (D).

From time to time, the peer server 42 will become disconnected from the proxy server 40, especially in home environments where users often shut down their PCs when not in use. In this case, the proxy server 40 cannot communicate to the peer server 42 to determine if the images 48′ in the cache 52 are still valid. Therefore, the proxy server 40 needs an updated set of the most current images and web page components surrounding those images before the peer server 42 goes off-line.

According to a further aspect of the present invention, this is handled via the synchronization server 54 (FIG. 2) and a synchronization protocol. Referring to both FIGS. 2 and 3, in operation, the user may initiate the synchronization protocol between the peer server 42 and the synchronization server 54 in step 80 prior to disconnecting the peer server 42. In a preferred embodiment, the user interface of the peer node 44 displays a peer synchronization icon or menu item that the user may select. Alternatively, the user may be prompted to perform synchronization, which the user may choose to accept or declined.

Once synchronization is invoked, in step 82 the peer server 42 uploads the timestamps 50 of all the images 48 to the synchronization server 54 via path (C). In step 84, the synchronization server 54 compares the uploaded timestamps 50 to the timestamps 50′ to determine if the cached images 48′ are current and whether there are any missing images in the cache 52. In step 86, the synchronization server 54 sends a request to the peer server 42 for any images identified as being modified or missing. In step 68, the peer server 42 retrieves and transmits the requested images 48 to the proxy server 40 via path (C).

In a preferred embodiment, synchronization is performed immediately upon request by the user. However, in an alternative embodiment, synchronization may be performed automatically in the background. That is, the proxy server 40 may be synchronized with the peer server 42 at the same time the proxy server 40 is serving images to the visiting computer 32. In this embodiment, synchronization may be performed when the peer server 42 first makes a connection to the proxy server 40. Background synchronization may also be performed when it is detected that the peer server's connection is idle. In either form of background synchronization, by time the user shuts down the peer server 42 after an active session, synchronization with the proxy server 44 may be more than likely complete.

In a preferred embodiment, the synchronization server 54 is a component of the proxy server 40. However, the synchronization server 54 may separate from the proxy server 40 and run on a separate computer.

As can be seen, an image is only transferred from the peer server 42 to the proxy server 40 via path (C) when it is not present in the cache 52, which is typically the case the very first time the image is request, and when the image in the cache 52 needs to be updated. However, other than the first time the image is requested, the image will be present in the cache 52, and the only data passed between the peer server 42 to the proxy server 40 via path (C) is the HTTP return code, which is only a few bytes, rather than megabytes to transfer the image. Due to this decrease in traffic between the peer server 42 to the proxy server 40, image viewing performance of the P2P network for guests is significantly increased.

FIG. 4 is a flow diagram illustrating the process of a peer server 42 registering with the photosharing peer-to-peer network 22 to make its serving capabilities assessable through a firewall 34. In a preferred embodiment, the P2P network 22 includes several proxy servers 40 a-n, referred to collectively as proxy server array 40, a peer server table 70, a registration server 72, and a DNS server 74.

The registration process begins in step 100, in which the peer node 44 passes its name to the registration server 72, the registration server 72 checks to make sure that the peer name is unique, and returns to the peer node 44 the name and IP address of the proxy server 40 to which it is assigned. In step 102, the peer node 44 registers its proxy server name and proxy server IP address with the DNS server 74. The DNS server 74 maintains a table of all peer names and their corresponding proxy IP addresses. In step 104, the peer node 44 registers the peer server's name and socket to proxy server 40 to which it was assigned.

In step 106, a user of the visiting computer 32 is notified that content (e.g., photos) exists on the peer server 42 for viewing. The notification could be implemented using several methods, but in a preferred embodiment, the user is notified via e-mail, with the e-mail including the URL of the content in the peer server 42. In step 108, the user of the visiting computer 32 receives the e-mail, and clicks on the URL. Using the peer name in the URL, the visiting computer 32 contacts the DNS server 74 to determine the identity of the proxy server 40 in which to send the request. The DNS server 74 responds with the IP address of the proxy server 40 assigned to the peer server 42. Given the proxy IP address, the web browser 30 of the visiting computer 32 sends an HTTP request to the proxy server 40 in step 110.

FIG. 5 is a diagram illustrating components of the proxy server 40 and the flow between the requesting web browser 30, the proxy server 40, and the peer server 42 to enable the web browser 30 to have HTTP access to the peer server 42 through the proxy server 40. In a preferred embodiment, the proxy server 40 includes multipleservlet threads 150, a registration manager 152, a peer manager 154, a peer MessageBox 156, and a peer packet manager thread 158.

The process begins in step 200 when the servlet thread 150 in the proxy server 40 receives the HTTP request in the form of a URL from the web browser 30. In step 202, the registration manager 152 checks the server table 70 (see FIG. 4) to determine if the peer server identified in the requesting URL is registered with the peer server 42, and if so, returns the corresponding peer socket. In step 204, the servlet thread 150 creates a peer request packet 160 from the HTTP request and then passes that packet to the peer manager 154.

FIG. 6A is a diagram illustrating the contents of a peer request packet 160. In a preferred embodiment, the peer request packet 160 includes a MessageBoxID 162, an HTTP URL 164, multiple HTTP headers 166, and an HTTP Post Data field 168. The MessageBoxID 162 is a unique identifier for correlating peer request packets 162, peer response packets 170, and peer message boxes 156. The HTTP URL 164 is the URL that was requested from the visiting web browser 30. The HTTP Headers 166 is the HTTP headers from the original request from the visiting web browser 30. The HTTP Post Data field 168 contains data for when the request is a POST command, and not a GET command.

Referring again to FIG. 5, in step 206, the peer manager 154 finds the socket connection to the peer server 42 and passes the peer request packet 160 to peer server 42. In step 210, the servlet thread 150 gets a peer MessageBox 156 from the peer manager 154 and blocks, waiting for response packets to arrive in the peer MessageBox 156.

In step 212, the peer node 44 receives the request packet 160, converts the packet 160 back into an HTTP request, and sends the HTTP request to the web server 28. In step 214, an HTTP response is sent from the web server 28 to peer node 44, which then takes the HTTP headers from the response, creates a peer response packet 170, and sends it back to the proxy server 40. The remaining portion of the HTTP response is broken up into 2K chunks in step 216 and sent to the proxy server 40 in successive peer response packets 170. In a preferred embodiment, the peer node 44 inserts a routing address with each peer response packet 170. Note that there can be several threads handling request from the proxy server 40. Therefore, the peer node 44 multiplexes those responses over the same response socket back to the proxy server 40.

FIG. 6B is a diagram illustrating the contents of a peer response packet 170. In a preferred embodiment, the peer response packet 170 includes a MessageBoxID 172, a packet size 174, a packet type 176, and a payload field 178. The MessageBoxID 172 is a unique identifyer for correlating peer request packets 162, peer response packets 170, and peer message boxes 156. The packet size 174 has to do with the fact that the response to the peer request packet 160 is sent back to the proxy server 40 in chunks. A packet size of 2K is used in the preferred embodiment. The individual packets are reassembled on the proxy server 40 to form the complete HTTP response, which is then returned to the visiting web browser 30. The packet type 176 indicates the type of data being returned in the payload field 178. Possible values include: [data, header, final packet]. The payload field 178 is the data portion of the peer response packet 170.

Referring again to FIG. 5, in step 218, the proxy server 40 receives raw bytes over the response socket and passes them to a peer packet manager 158 thread selected from a thread pool. In a preferred embodiment, there is only one peer packet manager thread per peer that is actively receiving requests 158 in the proxy server 40 170. In step 220, the peer packet manager thread 158 waits until there is a complete packet in its buffer, then routes the complete peer response packet 170 to the corresponding peer MessageBox 156. When the packet 170 arrives in the peer MessageBox 156, the corresponding servlet thread 150 wakes up and retrieves the complete peer response packet 170. In step 242, the servlet thread 150 converts the peer response packet 170 back into an HTTP response and then sends the HTTP response back to the requesting web browser 30. As disclosed herein, a combination of the proxy server 40 and the peer node 44 enable HTTP access to a peer server 42 located behind a firewall 34 by a visiting web browser 30.

The present invention has been described in accordance with the embodiments shown, and one of ordinary skill in the art will readily recognize that there could be variations to the embodiments. For example, while the preferred embodiment has been described in relation to a web-based peer-to-peer network, those skilled in the art would readily appreciate that the same principles can be applied to a conventional client-server environment where the client computer communicates directly with the peer server without utilizing the proxy server. Any variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims. 

1. In a network photosharing system having at least one peer computer coupled to a photosharing system server, a method for serving an image stored in the peer computer to a requesting computer, comprising: (a) caching copy of the image in the photosharing server; (b) in response to the photosharing server receiving a request from the requesting computer to view the image stored in the peer computer, sending a request from the photosharing server to the peer computer to determine if the image stored on the peer computer has been modified; (c) if a response from the peer computer indicates that the image has not been modified, transmitting the cached image from the photosharing server to the requesting computer; and (d) if the image has been modified, caching a copy of the modified image on the photosharing server, and transmitting the modified image from the photosharing server to the requesting computer.
 2. The method of claim 1 wherein step (b) further includes: sending the request to the peer computer as an HTTP request.
 3. The method of claim 2 wherein step (b) further includes: sending an HTTP If-Modified-Since header and a timestamp of the cached image as the request.
 4. The method of claim 3 wherein step (b) further includes: in response to the peer computer receiving the HTTP request, (i) comparing the timestamp for the cached image with a timestamp of the image stored on the peer computer; (ii) if the timestamps are different, responding by transferring the modified image to the photosharing server; and (iii) if the timestamps are not different, sending the response indicating that the image has not been modified.
 5. The method of claim 1 wherein step (a) further includes: (i) determining if the image stored in the peer computer is also stored in the photosharing server; and (ii) if the image is not stored in the photosharing server, transferring a copy of the image to the photosharing server for storage.
 6. The method of claim 5 wherein step (a) is performed a first time the image is requested.
 7. The method of claim 6 wherein step (a) is performed during a synchronization process between the photosharing server and the peer server.
 8. The method of claim 7 further comprising: once the synchronization process is invoked, uploading timestamps of the images stored on the peer server to the photosharing server, and comparing the uploaded timestamps to the timestamps for the images in the cache to determine if the cached images are current and whether or missing images in the cache.
 9. The method of claim 8 further comprising: sending a request from the photosharing server to the peer server for any images identified as being modified or missing, thereby synchronizing the images cached on the photosharing server with the images stored on the peer server.
 10. The method of claim 9 further comprising: performing background synchronization.
 11. The method of claim 9 wherein the photosharing server includes the synchronization server for performing the timestamp comparisons.
 12. The method of claim 1 wherein the photosharing server comprises a proxy server that provides the requesting computer HTTP access to the peer server when located behind a firewall.
 13. A network photosharing system having at least one peer computer coupled to a photosharing system server, comprising: (a) means for caching copy of the image in the photosharing server; (b) means responsive to the photosharing server receiving a request from the requesting computer to view the image stored in the peer computer, sending a request from the photosharing server to the peer computer to determine if the image stored on the peer computer has been modified; (c) means for transmitting the cached image from the photosharing server to the requesting computer if a response from the peer computer indicates that the image has not been modified; and (d) means for caching a copy of the modified image on the photosharing server if the image has been modified, and for transmitting the modified image from the photosharing server to the requesting computer.
 14. The system of claim 13 wherein the means for receiving the request sends the request to the peer computer as an HTTP request.
 15. The system of claim 14 wherein the means for receiving the request sends an HTTP If-Modified-Since header and a timestamp of the cached image as the request.
 16. The system of claim 15 wherein the means for receiving the request is responsive to the peer computer receiving the HTTP request for, (i) comparing the timestamp for the cached image with a timestamp of the image stored on the peer computer; (ii) if the timestamps are different, responding by transferring the modified image to the photosharing server; and (iii) if the timestamps are not different, sending the response indicating that the image has not been modified.
 17. The system of claim 13 wherein the caching means further includes means for: (i) determining if the image stored in the peer computer is also stored in the photosharing server; and (ii) if the image is not stored in the photosharing server, transferring a copy of the image to the photosharing server for storage.
 18. The system of claim 17 wherein the image is cached a first time the image is requested.
 19. The system of claim 17 wherein the image is cached during a synchronization process between the photosharing server and the peer server.
 20. The system of claim 19 wherein once the synchronization process is invoked, timestamps of the images stored on the peer server are uploaded to the photosharing server and compared the uploaded timestamps to the timestamps for the images in the cache to determine if the cached images are current and whether or missing images in the cache.
 21. The system of claim 20 wherein a request is sent from the photosharing server to the peer server for any images identified as being modified or missing, thereby synchronizing the images cached on the photosharing server with the images stored on the peer server.
 22. The system of claim 21 wherein synchronization is performed in the background.
 23. The system of claim 21 wherein the photosharing server includes the synchronization server for performing the timestamp comparisons.
 24. The system of claim 13 wherein the photosharing server comprises a proxy server that provides the requesting computer HTTP access to the peer server when located behind a firewall.
 25. A computer-readable medium containing program instructions for serving an image stored in the peer computer to a requesting computer in a network photosharing system where the peer computer is coupled to a photosharing system server, the instructions for: (a) caching copy of the image in the photosharing server; (b) in response to the photosharing server receiving a request from the requesting computer to view the image stored in the peer computer, sending a request from the photosharing server to the peer computer to determine if the image stored on the peer computer has been modified; (c) if a response from the peer computer indicates that the image has not been modified, transmitting the cached image from the photosharing server to the requesting computer; and (d) if the image has been modified, caching a copy of the modified image on the photosharing server, and transmitting the modified image from the photosharing server to the requesting computer.
 26. The computer-readable medium of claim 25 wherein instruction (b) further includes: sending the request to the peer computer as an HTTP request.
 27. The computer-readable medium of claim 26 wherein instruction (b) further includes: sending an HTTP If-Modified-Since header and a timestamp of the cached image as the request.
 28. The computer-readable medium of claim 27 wherein instruction (b) further includes: in response to the peer computer receiving the HTTP request, (i) comparing the timestamp for the cached image with a timestamp of the image stored on the peer computer; (ii) if the timestamps are different, responding by transferring the modified image to the photosharing server; and (iii) if the timestamps are not different, sending the response indicating that the image has not been modified.
 29. The computer-readable medium of claim 25 wherein instruction (a) further includes: (i) determining if the image stored in the peer computer is also stored in the photosharing server; and (ii) if the image is not stored in the photosharing server, transferring a copy of the image to the photosharing server for storage.
 30. The computer-readable medium of claim 29 wherein instruction (a) is performed a first time the image is requested.
 31. The computer-readable medium of claim 30 wherein instruction (a) is performed during a synchronization process between the photosharing server and the peer server.
 32. The computer-readable medium of claim 31 further comprising: once the synchronization process is invoked, uploading timestamps of the images stored on the peer server to the photosharing server, and comparing the uploaded timestamps to the timestamps for the images in the cache to determine if the cached images are current and whether or missing images in the cache.
 33. The computer-readable medium of claim 32 further comprising: sending a request from the photosharing server to the peer server for any images identified as being modified or missing, thereby synchronizing the images cached on the photosharing server with the images stored on the peer server.
 34. The computer-readable medium of claim 33 further comprising: performing background synchronization.
 35. The computer-readable medium of claim 33 wherein the photosharing server includes the synchronization server for performing the timestamp comparisons.
 36. The computer-readable medium of claim 25 wherein the photosharing server comprises a proxy server that provides the requesting computer HTTP access to the peer server when located behind a firewall. 